U.S. Pat. No. Re. 24,906 (Ulrich) concerns pressure-sensitive adhesive copolymer of about 88-97 parts of acrylic acid ester of non-tertiary alcohol, the alkyl groups of which have an average of 4-12 carbon atoms in the alkyl group, and correspondingly about 12-3 parts by weight of at least one modifying copolymerizable monomer such as acrylic acid, itaconic acid or acrylamide. A tape coated with the copolymer exhibits excellent adhesion and holding power, and the adhesive coating experiences no observable deterioration even after the tape has been stored for a number of years. Such tapes are widely used for a variety of purposes.
U.S. Pat. No. 3,728,148 (Pietsch et al.) teaches pressure-sensitive adhesive copolymer similar to that of Re. 24,906 except that the modifying copolymerizable monomer is an unsubstituted N-vinyl lactam, preferably N-vinyl-2-pyrrolidone, and is used in amounts from 10 to 30% by weight of total monomer, preferably 10 to 20% by weight. By this change in copolymerizable monomer, it is said that good electrical insulating properties are obtained compared to the corrosive effects on conductors of the copolymerizable monomers used in No. Re. 24,906. All self-adhesive properties, viz., adhesion, cohesion and gripping capacity, are said to be improved. The Pietsch patent teaches that the Fikentscher K-value of the copolymer, a measure of average molecular weight, should be from 50 to 100, preferably 60 to 90.
While the Pietsch and Ulrich patents disclose only polymerization in solution or in emulsion, U.S. Pat. No. 4,181,752 (Martens et al.) discloses and claims a process for preparing the same class of pressure-sensitive adhesive copolymers by subjecting a solvent-free coating of the liquid monomers to radiation having wavelengths of 3000-4000 A at a rate of 0.1 to 7 milliwatts per square centimeter of the exposed coating, while controlling the amount of radiation of shorter wavelengths to not more than about 10% of the amount of energy in the 3000-4000 A range. As compared to processes disclosed in the Ulrich patent, this process both reduces total energy requirements and eliminates pollution problems. N-vinylpyrrolidone is one of the modifying copolymerizable monomers used in the Martens patent.
In spite of the aforementioned teaching in the Pietsch and Martens patents, it is believed that no one has ever marketed a pressure-sensitive adhesive or adhesive copolymer of acrylic acid ester and an N-vinyl lactam such as N-vinylpyrrolidone. We surmise that there has been little or no commercialization due to inferior adhesive properties. Even though the Pietsch patent speaks of improved adhesive properties, the values given in the examples for "adhesive power" and "holding power" are considerably lower than those of pressure-sensitive adhesive tapes now on the market.
To improve the "holding power" of a pressure-sensitive adhesive, one must usually increase its cohesive strength. This usually is accomplished at the expense of tackiness and the ability to bond quickly, often called quick-stick.